A mass spectrometer such as a conventional Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) is configured to produce an ion from a to-be-analyzed sample using a device such as an electrospray ionization (ESI) source in the outside and then to implant the ion into a detection unit. Hence, the space between the ionization source and the detection unit should be opened, and the detection unit should be maintained at a very low pressure, i.e., ultra high vacuum pressure, differently from the ionization source under atmospheric pressure.
Accordingly, in order to implant the ion from the ionization source under atmospheric pressure while maintaining the detection unit in an ultra high vacuum state, a differential pumping system in which gas molecule conductance limit plates are installed for every vacuum chamber transmitting the ion is used. However, it is not possible to extend the length of an ion transmission portion because the ion transmission efficiency decreases rapidly as the length of the transmission path becomes longer, and thus it has been difficult to form an ultra high vacuum state in the detection unit.
FIG. 1 is a schematic view illustrating a differential pumping system according to a conventional technology. As illustrated in FIG. 1, according to a conventional differential pumping system, two vacuum pumps 10 are linearly arranged along a line in a vacuum chamber, and a conductance limit plate 11 is installed between the compartments in which the vacuum pumps 10 are arranged.
Here, the conductance limit plate 11 is a plate that has a small orifice at its center and the remaining portions of which are blocked. The conductance limit plate 11 passes only ions concentrated by an electric field and limits the movement of neighboring neutral gas molecules.
According to the differential pumping system as illustrated in FIG. 1, because the vacuum pumps for the differential pumping operation are sequentially arranged on a straight line, the length of the vacuum chamber is greatly extended as the number of the vacuum pumps is increased in order to form an ultra high vacuum state, so that it is not possible to maintain the length of an ion transmission portion.
That is, although it is possible to form a higher vacuum state, the decrease of the ion transmission efficiency cannot be avoided. As such, according to the prior art, because vacuum pumps are sequentially arranged on a straight line along a vacuum chamber, it has not been possible to simultaneously improve the degree of vacuum and the ion transmission efficiency.